1. Field Of the Invention
The present invention relates to a control system and method for a semi-automatic mechanical transmission system, including a manually operated shift selector, for automatically controlling the changing or shifting of transmission gear ratios on a vehicle, while leaving the vehicle driver or operator some residual measure of control. In particular, the present invention relates to a control system and method for the semi-automatic control of a mechanical change gear transmission including means for automatically executing automatically determined and displayed allowable driver selected transmission ratio shifts, including automatic control of the vehicle master clutch in all but start from stop situations. More particularly, the present invention relates to a method of controlling a semi-automatic mechanical transmission wherein a single downshift request made at a time when engine speed is below a predetermined reference engine speed is interpreted as a request for a downshift into the transmission ratio providing an engine speed approximately equal to the midpoint between maximum governed engine speed and peak torque engine speed.
2. Description of the Prior Art
Fully automatic transmission systems, both for heavy-duty vehicles such as heavy-duty trucks and/or coaches, and for automobiles, that sense throttle openings or positions, vehicle speeds, engine speeds, and the like, and automatically shift the vehicle transmission in accordance therewith are well known in the prior art. Such fully automatic change gear transmissions include transmissions wherein pressurized fluid is utilized to frictionally engage one or more members to other members or to a ground to achieve a selected gear ratio and automated mechanical transmissions utilizing electronic and/or pneumatic logic and actuators to engage and disengage mechanical (i.e. positive) clutches to achieve a desired gear ratio. Examples of such transmissions may be seen be reference to U.S. Pat. Nos. 3,961,546; 4,081,065 and 4,361,060, the disclosures of which are hereby incorporated by reference.
Such fully automatic change gear transmissions can be unacceptably expensive, particularly for the largest heavy-duty vehicles which are not typically sold in high volumes. Additionally, those automatic change gear transmissions utilizing pressurized fluid and/or torque converters tend to be relatively inefficient in terms of power dissipated between the input and output shafts thereof. Moreover, many vehicle operators like to control the selection of the gear ratios, particularly since they can see and/or know the nature of the road ahead and/or of the load being carried. Heavy-duty vehicles must often by accurately maneuvered into restricted spaces at slow speeds, wherein fully automatic transmissions tend to only engage a given low gear. Particularly at such times, when fuel metering to the engine is imperfect, as often happens temporarily in any vehicle system, the vehicle operator tends to have difficulty in performing delicate maneuvers with a fully automatic transmission, especially those not equipped with a manual clutch control pedal, unless the operator is also utilizing the vehicle brake with his left foot.
The above drawbacks have been minimized by providing a semi-automatic mechanical transmission control wherein automatic changing between a plurality of gear ratios is provided while normally permitting the driver to choose when to make a particular gear ratio change, whether to select the immediately consecutive ratio up or down or to skip one or more ratios, and when to slip, i.e. to incompletely engage, the vehicle master clutch in a low or crawler gear. Furthermore, the semi-automatic control system prohibits engagement of a driver selected higher ratio, i.e. an upshift, that would cause the engine speed to decrease below the idle speed, i.e. would cause stalling and/or over-stressing at low speeds of the engine, and of a lower ratio, i.e. a downshift, if such a change would cause overspeeding of the engine. It is noted that with manual mechanical transmissions such overspeeding of the engine can occur due to a improper downshift even though the engine speed may be governed against accelerating above the governed engine RPM. Examples of such a semi-automatic transmission control can be seen by reference to allowed U.S. patent application Ser. No. 755,628 filed July 16, 1985, now U.S. Pat. No. 4,648,290.
While the above-described semi-automatic mechanical transmission control does provide a very desirable semi-automatic control, the control was subject to improvement as, if the driver or operator desired a smooth shift into a strong pull-away gear after slowing of the vehicle sufficiently to cause engine speed in the current ratio to fall below an acceptable torque level, the driver was required to identify the desired ratio, to calculate the number of downshifts from the currently engaged ratio to the desired ratio and to then move the selector lever that number of times in the downshift direction.